Live attenuated varicella vaccine was approved in March, 1995 and has been recommended for routine use in susceptible, healthy persons 12 months of age. In the first four years of this study, we have demonstrated that the effectiveness of varicella vaccine in clinical practice was 85 percent overall and 97 percent against moderate or severe disease. Vaccinees who developed varicella generally had very mild disease. However, there continues to be uncertainty about the long-term effectiveness of the vaccine. The primary purpose of the proposed study is to assess the effect of time since vaccination and of age at the time of vaccination on the effectiveness of varicella vaccine as it is used in actual practice. This will be a continuation of a case-control study in which the cases will be immunocompetent children from 13 months to 18 years of age who develop varicella. The cases will be identified through active surveillance of pediatric practices in Connecticut. A research assistant will visit the home of each potential case subject on the third day of the rash to assess the severity of the illness. In addition, material from the lesions will be tested with the polymerase chain reaction test to confirm the presence of varicella-zoster virus and to determine whether it is wild-type or vaccine-type virus. The controls will be immunocompetent children without a history of varicella who are matched to the cases (2 controls/case) by both age and pediatric practice. The medical records of both the cases and the controls will be reviewed to determine whether the child had received varicella vaccine. The effectiveness of the vaccine will be estimated from the matched odds ratios. The effects on the effectiveness of the vaccine of both age at the time of vaccination and time since vaccination will be assessed by conditional logistic regression. The estimates will be adjusted for possible confounders with the use of conditional logistic regression. In addition, differences in the clinical severity of varicella among vaccinated and unvaccinated cases will be assessed, as will whether the severity of disease among vaccinees is affected by either time since vaccination or age at the time of vaccination. This study will provide important information about the effectiveness of varicella vaccine over time that is critical for assessing the cost-effectiveness of the vaccine and the possible need for booster doses, and will be useful both for educating physicians and for counseling parents about having their children vaccinated.